The housing of the lower drive unit, or gear case, of modern outboard motors is configured to include a plate that projects above the propeller and helps to reduce the amount of surface air and/or exhaust drawn into the propeller blades. This antiventilation plate typically includes a dependent trim or torque tab that may assist in controlling steering imbalance or torque.
Some steering torque may occur when an engine with a right-handed propeller is trimmed in, because this causes the propeller shaft to tilt upwardly at the aft end, which increases the pitch of the downward blade on the right side of the shaft. In recent years, the manufacturers of marine outboard or stern drive motors have increased the recommended mounting height for their engines. The raised propeller may break the surface of the water, especially when the boat is trimmed in. Under these circumstances, a right-handed propeller tends to push the stern to the right, rotating the boat along with it in a right-handed turn unless counteracted at the wheel. This so-called “propeller walk” or “paddle-wheel” effect is not limited to propellers on engines raised above the standard installation height. It may also occur when a boat is operated at high speed, in waves or if the boat pitches. It may be a moderate effect, or it may outweigh all other causes of steering torque, and it may adversely affect steerability.
Steering torque is normally counteracted by using the trim tab to help steer the engine back to a forward course. However, since the trim tab sits on the antiventilation plate above the propeller, in a raised engine the trim tab may also have been raised above the water line, or to a height that impairs its effectiveness in altering steering torque.
The gear case also projects below the propeller in the form of a dependent fin. This so-called skeg portion is thus always positioned below the propeller in the water. The skeg serves to protect the propeller from striking submerged objects or the bottom surface. It may also function as a fixed rudder to assist in steering.
A variety of torque tabs or plates have been developed for attachment to the skeg to compensate for steering torque. Because these skeg tabs are subject to substantial force of water from the rotating propeller blades, they cannot be adhesively attached to the skeg. Instead, they must be fixed in place by means of fasteners inserted into holes that are drilled into the body of the skeg. Placement on one side of the skeg addresses right-hand steering torque, while placement on the other side of the skeg addresses left-hand steering torque. Because placement is dependent on a number of variables such as propeller configuration, engine mounting height, engine trim angle, engine horsepower and speed, it is difficult to determine a proper placement for the tab or plate prior to attachment to the skeg. Repositioning of the skeg tab, either to switch handedness or to adjust its location on the same side, requires removal of the fasteners, drilling new holes in the body of the skeg, and refastening the tab. This procedure is cumbersome, time consuming, and impractical. Moreover, drilling additional fastener holes in the skeg weakens it, and may ultimately result in damage that necessitates replacement of the skeg.
Because of the downward projection of the skeg, it is subject to damage and breakage from encounters with submerged objects such as stumps, gravel, rocks and the like. Damaged metal skegs may be repaired by welding, but such repair is labor intensive, costly and once repaired, the skeg remains subject to continued damage. Polymer and composition skegs cannot generally be mended, and alternate replacement of the entire lower gear case is not cost effective. Thus, damaged skegs can commonly be restored by fastening a replacement shell over the stub of the original skeg.
There is a need for a marine skeg having an adjustable vane structure for reducing or eliminating steering torque, that is positioned on the gear case at or below the level of the propeller so as to remain submerged with the propeller and maintain steering torque reduction regardless of the mounting height of the engine on the boat, the trim of the boat, or the water conditions, that can be easily adjusted without the need for removal of the skeg, that has an adjustable vane structure that, when fixed, helps to act as a fixed rudder to assist in steering control and that can be adjusted to eliminate either right-handed or left-handed steering torque, that further can be repeatedly readjusted and fixed in place to compensate for different boating situations, that can be repeatedly readjusted without drilling additional holes in the skeg, that can be configured as an integral part of the gear case of a motor or as a replacement skeg for fitting over an existing skeg or skeg stump, and that is economical to construct, durable and simple to operate.